1. Field of the Invention
The present invention relates to a rolling mill stand having rolls that are axially slidable with respect to each other, more particularly, to work rolls or intermediate rolls having a seventh order polynomial surface profile which together form an adjustable gap with a sixth order profile.
2. Prior Art
Strip product such as aluminum is typically rolled in a four high or six high rolling mill stand. Recently, the demand for thin aluminum strip product has increased, particularly for the beverage can industry. For such applications, the strip must be hot rolled to as thin as 0.090 inch with minimal variations in flatness. Flatness defects should be avoided and the strip should have a constant thickness over its entire length. To avoid unevenness of the strip, it is necessary to roll the strip uniformly over its width, so that internal stresses are avoided which could lead to undesirable undulations in the middle area, the edge area or the quarter area of the strip. Such internal stresses typically result in edge cracking during hot rolling and subsequent cold rolling which requires that sections of the coil with large cracks be cut away and scrapped. If edge cracking occurs in the middle of a coil, the entire coil may need to be scrapped.
Strip flatness defects are due in part to the forces exerted on the rolls from the strip, referred to as bending deflections, and alterations in the diameter of the rolls which develop across the length of the rolls. These alterations are caused by the force of the strip flattening the rolls and by thermal expansion of the rolls which creates a thermal camber on the roll surfaces. The mid-point of the roll is the hottest, hence the thermal expansion of rolls is greatest at the mid-point of the rolls and decreases towards the ends of the rolls. The resulting roll gap profile is uneven across the length of the rolls which creates uneven rolling across the width of the strip. Uniform rolling can only occur if the roll gap profile under load is properly adjusted by means of adjusting mechanisms.
One such adjusting mechanism is a bending jack. Bending jacks are applied to the neck of the roll to exert a force to compensate for the bending deflections and thermal camber. The jacking force is designed to counter the vertical shift in the roll surface at the roll mid-point by bending the ends of the roll, so surfaces of the ends of the roll are in the same plane as the roll mid-point. While bending jacks compensate for bending deflections very well because they are both parabolic in their functional form the roll gap profile may still not be sufficiently corrected to produce thin strip with acceptable flatness in situations where the thermal profile of the work rolls has a large magnitude and steep drop off at the edges of the strip. In these situations, use of parabolic actuators such as bending jacks and traditional ground work roll crowns will not provide the required compensation and quarter buckle flatness defects will appear in the sheet. Additionally, use of bending actuators with fixed roll crowns may not provide adequate adjustment range for mills that process a wide range of products of different material hardness and strip width.
One system for accommodating the characteristics of various rolled products and varying thermal crown on the work rolls is described in U.S. Pat. No. 4,881,396. Axially slidable rolls are shaped in such a way that the effect resulting firom the contours of two rolls can be determined by the relative axial displacement of the rolls. A roll gap of various parabolic and quartic shapes can be created by adjusting the shift position of the rolls to tailor the mill to the characteristics of the product being rolled. Bottle shaped rolls may be operated to provide a continuously variable crown (CVC) capable of compensating for parabolic bending over the entire length of the roll bodies and the effect of thermal camber build-up if the magnitude and steepness are not too great and the strip is fairly thick. However, while such CVC rolls provide more flexibility and actuator range to roll a broad product mix, they may not completely compensate for the thermal profile of the roll and still may produce wavy sections of the strip.
The '396 patent further describes using rolls having profiles described as fourth order polynomials to reduce the waviness in the edge or quarter areas of the strip. Such fourth order polynomial roll gap profiles have improved strip quality yet still have not completely eliminated quarter buckle flatness defects particularly when the temperature of the rolls is high during high speed rolling and/or high reduction. In an multi-stand aluminum hot rolling mill, the strip typically enters the first mill stand at about 750.degree. F. and exits the mill stand at about 650.degree. F. The rolls are continually sprayed with a coolant such as a water and oil mixture. The centers of the rolls are typically about 215.degree. F. and the edges of the roll are about 190.degree. F. The temperature differences across the length of the roll (the temperature profile of the roll surface) causes varying expansion in the roll surface and hence, varying flatness in the strip. The temperature profile of the roll surface temperature changes over time as the mill stand heats up with use and eventually reaches a steady state condition.
Accordingly, a need remains for a system to compensate for the varying thermal expansion of the rolls in a strip mill stand to obtain a strip which is fiee fiom quarter buckle defects.